Muon dedx

1
A STUDY OF EMITTANCE MEASUREMENT AT THE ILC
L. Jenner, Liverpool University, Cockcroft
Institute, Daresbury Laboratory, Warrington,
U.K. D.Angal-Kalinin, CCLRC,ASTeC, Cockcroft
Institute, Daresbury Laboratory, Warrington,
U.K. G. Blair, I. Agapov, J. Carter, L. Deacon,
John Adams Institute at RHUL, London, U.K. M.
Ross, A. Seryi, M. Woodley, SLAC, Stanford,
California, U.S.A.

• At the ILC, the luminosity will depend on
  maintaining the emittance (especially in the
  vertical plane) delivered by the damping rings.
  An important part of this challenge is to remove
  any x-y beam couplings that arise from the linac
  and to measure the emittance.
• We propose to use laser-wires (LWs) in the Beam
  Delivery System (BDS) to measure the beam sizes
  for both emittance diagnostics and coupling
  correction.
• The accuracy of the beam size measurement
  depends upon several factors such as errors from
  beam jitter, spurious dispersion functions,
  coupling of the beams, laser pointing stability
  and the beta functions at the LW locations.
• The effects of these errors on the emittance
  measurement accuracy and its implications for the
  skew correction procedure are considered.

The difference in reconstruction for 5 and 20
error in the measurement at the LW.

• The emittance is measured by the LWs
• This information is iteratively fed-back to the
  orthogonal SQs such that the emittance reaches a
  minimum.
• The accuracy of this procedure is in part
  dependent on how well the true emittance can be
  reconstructed from the laserwire measurements.

Df x,y
The reconstructed emittance for different values
of the measurement error.

• A traditional wire scanner cannot withstand the
  thermal load of a 250 or 500 GeV beam, as is
  expected at the ILC.
• A laserwire can make a non-invasive measurement
  of a beam by bringing a laser into collision with
  it and then measuring the forward-scattered
  Compton photons and electrons, see right.

Optical layout of the proposed skew correction
and emittance measurement section at the ILC.
LW-Laser Wire, SQ-Skew Quadrupole

• The skew correction procedure works by iterating
  through the SQs and scanning across small range
  B-field settings, until the measured emittance at
  the LWs is minimised, see right.
• If the resolution of the measurement is too
  poor, the process can diverge.
• This problem can often be compensated for in part
  by making more measurements, or by increasing the
  number of steps in the scan both are time
  consuming undertakings.

• LW Compton events have been simulated using a
  dedicated full-simulation program (BDSIM), in
  order to determine the energy losses along the
  beam-line and to identify possible locations of
  Compton detectors.
• For a 250 GeV beam and using the baseline ILC
  BDS optics, approximately 98 of the Compton
  scattered photon energy exits at the downstream
  energy diagnostics chicane, shown left.

• The contributions of measurement errors on the
  beta functions, spurious dispersion, beam jitter,
  laser spot size, laser pointing error etc to the
  total measurement error.
• Considering the optimistic value of the
  measurement error is 23.8, it can be seen that
  in general the procedure works satisfactorily.

• CONCLUSIONS
• This study is still preliminary, however it is
  clear that the machine-related errors are
  significant and may dominate those coming from
  the LW itself further work is necessary.
• The extraction of the LW signal is also under
  investigation, using full simulation tools.
• The energy-diagnostics chicane provides a good
  location for a detector for the Compton scattered
  photons, where a very large signal will emerge.

• Initially the beam is set up such that the ratio
  of the projected (E2) to intrinsic (Ey) emittance
  is 3.8.
• This ratio is arrived at from linac simulations
  and representative of the expected quantity of
  x-y coupling in the ILC beam.
• After two scans of each skew quadrupole, the
  ratio is about 1.3. The error bars here are large
  due to the large measurement error.

Up to about 113 TeV of energy per bunch will
reach the detector which will require installing
shielding. In principle the scattered electrons
can also be detected downstream of the LW. The
energy loss due to these electrons is shown.